The invention relates to a process for reducing the opacity of plume released to the atmosphere from large-scale combustors, such as the type used industrially and by utilities to provide power and incinerate waste. According to the invention, plume opacity is mitigated, preferably while improving combustion and/or reducing slag and/or corrosion. The invention achieves one or more of these desired results through the use of various combinations of combustion catalysts, slag modifiers, targeted in-furnace injection, and/or in-body injection.
The combustion of carbonaceous fuels, such as heavy fuel oils, coals, refinery coke, and municipal and industrial waste, typically produces a plume arising from the smoke stack and can have opacity ranging from low to high. In addition, these fuels contain slag-forming materials, and can generate corrosive acids, and unburned carbon, that in combination have a relatively negative effect on the productivity of the boilers, and can corrode the environment and pose a health risk.
Plume is a problem from an aesthetic standpoint as well as an environmental one. Plume can be objectionable in and of itself and is expensive to treat by conventional technology. The negative effects of plume are considered to be related to the opacity of emissions from power plants. Plume opacity is measured in percent. Simply, the greater the opacity, the more the background behind the plume is obscured and the less light can come through the plume. If none of the background is obscured, then the opacity is 0%. If the entire background is obscured, then the opacity is 100%.
The visibility impairment effects of power plant plumes can be grouped into three categories. The first, opacity, occurs very near the stack and is determined by EPA Reference Method 9 is found in 40 CFR Part 60, Appendix A. It was adopted as a visible emissions inspection method in an effort to standardize the training and certification of observers and to ensure that reliable and repeatable opacity observations could be conducted anywhere in the United States. The second, plume blight, occurs at distances from 2 km to 1 day's travel downwind. Blight happens before the plume has been dispersed so widely that it is indistinct from the background. Regional haze is the effect of the plume on a broader scale and is obviously of critical concern. Coal and oil fired power plants, especially, produce small particles in plumes from when sulfur dioxide (SO2) is oxidized to sulfur trioxide (SO3) inside a furnace and boiler, condenses with water (H2O) at lower temperatures to become suspended sulfuric acid aerosol particles. SO3 also reacts with alkali metals to form various sulfates. Sulfate particles can significantly contribute to the concentration of very fine particle matter (PM2.5), which is associated with health as well as reduced visibility. Desulfurization, e.g., flue gas desulphurization (FGD), of the entire effluent can be used to decrease plume from coal-fired boilers by decreasing the overall SO2 content of the effluent. The invention, by decreasing plume opacity, directly affects opacity and is believed to greatly reduce an individual plant contribution to the other two categories of visibility impairment.
While plume opacity is of concern from an external pollution position, slagging and some of the other problems caused by combustion can affect efficiency—therefore, economics, which are a severe threat to older power plants, especially, where efficiency is required for pollution controls to be affordable for maintaining the plants in operation. Slagging deposits are sometimes extremely difficult to remove by conventional techniques such as soot blowing. Slag buildup results in a loss of heat transfer throughout the system, increases draft loss, limits gas throughput and is a factor in tube failure due to erosion from excessive sootblowing. A variety of other procedures are known for adding treatment chemicals to the fuel or into the furnace in quantities sufficient to treat all of the ash produced, in the hope of solving the slagging problem. Typical chemicals include magnesium oxide and magnesium hydroxide for the above reasons and various combustion catalysts, such as copper, iron, calcium, to improve the burning of the fuel.
Corrosion, typically occurs to a greater degree at the cold end of the combustor, and can create maintenance costs that are desirably avoided. Acid gases and deposits can often be controlled by the addition of chemicals to the combustion chamber or the fuel. The introduction of chemicals in this manner is often very inefficient and increases the amount of ash that must be disposed. Corrosion control is too often a choice between polluting byproducts.
The art has endeavored to solve slagging and/or corrosion problems by introducing various chemicals, such as magnesium oxide or hydroxide. Magnesium hydroxide has the ability to survive the hot environment of the furnace and react with the deposit-forming compounds, raising their ash fusion temperature and thereby modifying the texture of the resulting deposits. Unfortunately, the introduction of the chemicals has been very expensive due to poor utilization of the chemicals, much simply going to waste and some reacting with hot ash that would not otherwise cause a problem. U.S. Pat. No. 5,740,745 and U.S. Pat. No. 5,894,806 deal with this problem, by introducing chemical in one or more stages to directly address predicted or observed slagging and/or corrosion.
The presence of unburned carbon in the ash is an indication that combustion is not efficient and can cause operational problems. Increasing the amount of air used for combustion can reduce carbon in the ash, often referred to as LOI carbon (for loss on ignition, denoting a weight loss of ash due to combustion of its carbon content). This can be effective in some situations, but the use of excess air always decreases boiler efficiency. Also, excess air increases SO2 to SO3 conversion, causing additional acid aerosol plume and may also increase NOx levels. The use of combustion catalysts can also be effective in some cases; however, combustion catalysts cannot always be used effectively or efficiently due to fuel and/or equipment limitations. Among combustion catalysts proposed in the art are the metal compounds in the form of basic metal salts, generally calcium, iron, copper and magnesium compounds. Generally the metal compounds are delivered as metal salts. The anionic portion of the salt can be hydroxyl, oxide, carbonate, borate, nitrate, etc. Carbon in the ash can decrease commercial value of the ash, which can be used in concrete if the LOI can be reduced to less than 2%.
The art is in need of a process that can efficiently deal with plume, while preferably permitting efficient combustion with lower LOI carbon, lower excess air, lower CO, and/or lower NOx, and/or controlling slag, and/or corrosion.